1. Field of the Invention
This invention relates to a device for forming an electrostatic latent image used in electrostatic recording. The invention particularly relates to an electrostatic latent image forming device adopting ion current control.
2. Description of the Prior Art
A conventional electrostatic latent image forming device adopting ion current control is shown in FIG. 5. Driving electrodes 51 are disposed parallel to each other on an insulating substrate 50, while control electrodes 54 intersect the driving electrodes 51 through an insulating layer 52. A matrix is formed by the driving electrodes 51 and the control electrodes 54. Two thin electrodes 55 formed parallel to each other are connected to each control electrode 54. A space 56 for generating ions exists between the electrodes 55. A plate-like screen electrode 58 is disposed on the control electrodes 54 through an insulating layer 57, and the screen electrode 58 is, as shown in FIG. 6, provided with circular openings 59 for deriving ions only at positions corresponding to the space regions 56. An opening 60 is located in the insulating layer 57.
The electrostatic latent image forming device applies a high-frequency high voltage between the driving electrodes 51 and the screen electrode 58. The image forming device applies ion control voltage to the control electrodes 54, and applies d.c. voltage to the screen electrode 58. Creeping corona discharge occurs between the driving electrode 51 and the control electrode 54 in the space 56. Ions generated by creeping corona discharge are accelerated or absorbed in an electric field generated by the control electrode 54 and the screen electrode 58 to control ion emission and form an electrostatic latent image.
Thinly separated natural mica is used for the insulating layer 52. A thin plate made of copper, stainless steel or another conductive electrode material is laminated onto the surface of the natural mica, and each conductive thin plate is then photoetched to form the driving electrodes 51 and the control electrodes 54.
The prior art electrostatic latent image forming device is limited by the use of natural mica. Only homogeneous natural mica of the best quality with a designated size and thickness can be used. The availability of high quality, natural mica is limited, and mass production is difficult. If there is a flaw or a defect in the insulating layer, either creeping corona discharge will not occur, or dielectric breakdown will occur. In either event, stable discharge characteristics are not obtained. Further, if natural mica is used as an insulating layer, skilled craftsmanship for separating natural mica to a fixed thickness and for cutting natural mica to a designated size is required. Therefore, mass production is difficult.
A pressure sensitive adhesive may be used to laminate electrode material onto the surface of the natural mica. However, the pressure sensitive adhesive lacks heat resistance and discharge resistance, and, therefore, a manufactured electrostatic latent image forming device is unstable and unreliable.